The three-dimensional structures of folic acid antagonists and other inhibitors of the enzymes dihydrofolate reductase (DHFR) and thymidylate synthetase (TS) will be determined by X-ray crystallography. Comparison of these compounds stereochemical similarities and differences and collaborative data-sharing with crystallographers working on the structures of the two enzymes will provide insight into the stereochemical determinants of substrate and inhibitor binding to DHFR and TS. Since these enzymes are the targets of two of the most widely used cancer chemotherapy drugs, methotrexate and 5-fluorouracil, the information we accumulate will aid the process of systemic development of new anticancer drugs with better therapeutic indexes. Two other classes of anticancer drugs will also be investigated, analogues and pre-activated derivatives of the cyclophophamide family of alkylating agents and conformationally restricted analogues of the antimetastatic agent ICRF-159. These studies will continue projects that have already provided valuable data and aided our understanding of the chemical and biological properties of these drugs.